The present invention generally relates to polishing a surface of a workpiece. More particularly, the invention relates to improved methods and apparatus for in situ monitoring of a wafer during chemical mechanical polishing (CMP).
Chemical mechanical polishing or planarizing a surface of an object may be desirable for several reasons. For example, chemical mechanical polishing is often used in the formation of microelectronic devices to provide a substantially smooth, planar surface suitable for subsequent fabrication processes such as photoresist coating and pattern definition. Chemical mechanical polishing may also be used to form microelectronic features. For example, a conductive feature such as a metal line or a conductive plug may be formed on a surface of a wafer by forming trenches and vias on the wafer surface, depositing conductive material over the wafer surface and into the trenches and vias, and removing the conductive material on the surface of the wafer using chemical mechanical polishing, leaving the vias and trenches filled with the conductive material.
A typical chemical mechanical polishing apparatus suitable for planarizing the semiconductor surface generally includes a wafer carrier configured to support, guide, and apply pressure to a wafer during the polishing process; a polishing compound such as a slurry containing abrasive particles and chemicals to assist removal of material from the surface of the wafer; and a polishing surface such as a polishing pad. In addition, the polishing apparatus may include an integrated wafer cleaning system and/or an automated load and unload station to facilitate automatic processing of the wafers.
A wafer surface is generally polished by moving the surface of the wafer to be polished relative to the polishing surface in the presence of the polishing compound. In particular, the wafer is placed in the carrier such that the surface to be polished is placed in contact with the polishing surface and the polishing surface and the wafer are moved relative to each other while slurry is supplied to the polishing surface.
In order to determine when the wafer has been polished to its desired degree of flatness, the front surface of the wafer must be monitored during the planarization process. Many end-point detection systems rely on the change of the surface structure of the wafer as an overlying layer is removed. For example, a change in friction between the wafer and the polishing pad indicates end-point in U.S. Pat. No. 5,036,015, the analysis of reflected acoustic waves measures wafer thickness in U.S. Pat. No. 5,240,552, and special electrodes under the polishing pad combined with an electrically grounded polishing table and use of a conducted slurry are used to measure dielectric thickness in U.S. Pat. No. 5,337,015.
Ex-situ methods are also used to determine end-point. For example, wafers may be removed from the polishing apparatus so that flatness can be measured using a spectroscopic device, which measures oxide film thickness, before being replaced in the polishing apparatus for polishing to the desired endpoint. This process requires that the wafer be removed from polishing before the expected endpoint so that overpolishing does not occur.
Still other endpoint detection methods, such as that described in U.S. Pat. No. 5,081,796, move the wafer over the edge of the polishing pad to rapidly measure the oxide layer using techniques such as laser interferometry. However, with this method, moving a portion of the wafer over the edge of the pad results in the wafer failing to receive uniform polishing at all times.
More recent end point detection methods carry out optical detection of a wafer surface condition during polishing by utilizing a polishing pad having a portion thereof, or window, constructed from an optically clear polymer. The clear polymer window has no ability to absorb or transport slurry particles and is also transparent to the light beam being used to detect the wafer surface condition by optical methods. The process for making an apparatus capable of carrying out this type of endpoint detection method generally includes 1) making one or more holes in the polish pad at the locations for utilizing endpoint detection probes, 2) casting an optically clear liquid or gel adhesive polymer in the holes in the pad, 3) curing and finishing the cast polymer, 4) installing the pad on the platen of a CMP tool with transparent windows in the pad aligned to probe locations, and 5) installation of probes under the windows.
However, shortcomings of this most recent endpoint technology include the long and somewhat costly process of producing the transparent windows in the pads as well as potential manufacturing issues which may need to be addressed in the event that stacked polishing pads are used for polishing. Moreover, if curing of the transparent windows in the pads occurs at a temperature higher than room temperature, the adhesive layer on the pad may exhibit possible adverse effects. Finally, both logistic and quality issues may arise when there are multiple pad manufacturers.
Accordingly, there is a need for an in situ endpoint detection method and apparatus which is accurate as well as reliable and less costly.
The present invention provides improved methods and apparatus for chemical mechanical polishing of a surface of a workpiece that overcome many of the shortcomings of the prior art. While the ways in which the present invention addresses the drawbacks of the now-known techniques for chemical mechanical polishing will be described in greater detail hereinbelow, in general, in accordance with various aspects of the present invention, the invention provides an apparatus for end-point detection which includes a probe having an end for emitting and receiving light and a transparent plug mounted over the probe which fits into an opening in a polishing pad so that a polishing endpoint can be detected while polishing a workpiece.
In accordance with an exemplary embodiment of the present invention, the endpoint detection apparatus also includes a support member located beneath the plug and about an outer circumference of the probe. In order to perform endpoint detection using a CMP platen and polishing pad with this exemplary embodiment, the probe is positioned so that the support member fits into an opening within the platen and the plug fits into an opening within the polishing pad. The top of the transparent plug is positioned coplanar with the top surface of the polishing pad while the support member forms a sealable fit within the platen so that slurry does not leak through the platen during polishing.
In one aspect of the endpoint detection system of the present invention, the support member is attached to the probe such that it is in slidable engagement with the probe along a length of the probe. Accordingly, when the probe is elevated to position the plug within the opening in the polishing pad, the support member can be pushed upward toward the plug so that the support member fits securely and snugly within the opening in the platen.
In yet another aspect of the present invention, the plug and/or probe and/or support member may be removable and replaceable. The plug is preferentially comprised of a transparent polymer material that can transmit light signals while the support member is preferably comprised of a conformable polymer that is chemically resistant to slurry.
In still another aspect of the invention, a rigid sleeve member may be contained within the plug for receiving the end of the probe.
It will be understood by those skilled in the art that the endpoint detection system of the present invention is capable of being utilized in any polishing system which employs a polishing pad secured to a platen for polishing.
In accordance with another embodiment of the invention, a method for detecting endpoint during CMP is presented which includes 1) selecting a probe having an end for emitting and receiving light, 2) mounting a transparent plug over the end of the probe, 3) inserting the plug through an opening in a polishing pad so that the top of the plug is recessed or coplanar with respect to the top of the polishing pad, 4) polishing a workpiece against the top surface of the polishing pad, and 5) transmitting and receiving an optical signal through the probe to determine an endpoint for polishing.
In accordance with another embodiment of the invention, the above described method may also include the step of positioning a support member into an opening in a platen used to retain the polishing pad so that the support member forms a seal with the opening in the platen.